Noise estimator for seismic exploration

ABSTRACT

A method of generating a noise estimate during seismic surveying, including measuring noise energy having a plurality of frequencies in a second time interval. The second time interval having a second start time delayed from a first start time of a first time interval during which a first plurality of reflected seismic signals are present. The plurality of reflected seismic signals having the plurality of frequencies. The second time interval approximately concurrent with the first time interval to measure one of the plurality of frequencies of the noise energy different than one of the plurality of frequencies of the first plurality of reflected seismic signals that are present. A noise estimate is generated based on the noise energy measured.

This application is a continuation application of U.S. patentapplication Ser. No. 09/340,274 entitled “NOISE ESTIMATOR FOR SEISMICEXPLORATION,” filed on Jun. 25, 1999 now U.S. Pat. No. 6,366,857.

FIELD OF THE INVENTION

This invention relates to the field of seismic exploration and, morespecifically, to a method and apparatus for noise estimation in seismicsurveying.

BACKGROUND

In seismic surveying, acoustic energy waves are transmitted into theearth in order to map subterranean geological structures by sensingreturned acoustic energy waves reflected from those geologicalstructures. Land seismic surveys are commonly performed using vibroseistrucks that provide the source of the acoustic energy. The vibroseistrucks generate (“vibrate”) the acoustic energy waves at predeterminedvibrator points (“VP”) that are usually marked with stakes that havebeen placed by surveyors. During operations, the vibroseis trucksnavigate from point to point using these survey stakes.

The acoustic energy wave, known as a chirp sweep, vibrated by the trucksis swept in frequency over a period of time. A typical chirp may sweepfrom 5 to 150 Hertz (Hz) and last for 15 seconds. The subterraneangeological layers create changes in the chirp due to refractions,reflections, and diffractions at the boundaries of changes in acousticimpedance of each subterranean layer. Some of these altered acousticenergy waves, known as echoes, return to the earth's surface to besensed by seismic detectors. The arrival time of the echoes at theseismic detectors depends mainly on the depth of the subterranean layersreflecting the chirp. A listening time window is used to capture thereturn echoes down to the depth of interest. The echoes are compressedby correlating with the chirp sweep. The arrival times of the compressedechoes are used to generate imaging data of the subterranean structure.

One problem with seismic surveying operations is that the presence ofambient noise during the listening time window may drown out the echoesto be sensed by the seismic detectors. Ambient noise may be generated bysources in the area being surveyed such as wind tugging on grass orvehicular traffic. The imaging data process requires a minimumsignal-to-noise ratio (SNR) for the data to be of sufficient quality forsurveying use. If the ambient noise level is too high, then surveyingoperations may have to be halted until the ambient noise level falls toan acceptable SNR level. As such, in order to obtain an in-fieldestimate of the data quality, a passive measurement of the ambient noiselevel is made.

FIG. 1A illustrates a prior art sequential sweep operation that uses abroad band energy detector to measure the total noise energy across theentire frequency band of interest. The broad band noise estimationoccurs in between the end of a prior listening region and the start of anew chirp sweep. One problem with such a system is that in order toobtain an accurate noise estimate, a dead time (e.g., 2 seconds shown)when no sweeps occur is necessary for broad band energy detection. Thedeadtime for noise estimation adds to the overall cycle time of seismicsurveying operations.

For example, as illustrated in FIG. 1A, a single chirp sweep (15seconds), plus listening time (5 seconds), plus noise estimation (2seconds) may take 22 seconds. If 1000 VPs are made in a day, thenapproximately 33 minutes are used in a non-productive mode listening fornoise. Assuming a 12 hour working day, this translates to approximately5% of the available work time used for noise estimation.

FIG. 1B illustrates a prior art slip sweep operation that also uses abroad band energy detector to measure noise energy. In slip sweepoperations, multiple groups of vibroseis trucks are used in which agroup starts sweeping without waiting for the other groups' sweep to becompleted. The sweeps of the different groups occur in non-overlappingfrequency-time windows such that no two groups are sweeping in the samefrequency at the same time. For example, sweep 2 from a second group isstarted at time, ts, as soon as the start frequency, fs, of the previoussweep 1's echo region has completed.

As with sequential sweep operations, a slip sweep operation using abroad band noise estimator requires a dead time (e.g., 2 seconds shown)when no sweeps occur for broad band energy detection in order to obtainan accurate noise estimate. The noise estimate occurs in between the endof the last echo region of a vibroseis group's sweeps and the start of anew chirp sweep in a new vibroseis group's sweeps. One problem withusing a broad band noise estimator is that the operations of allvibroseis groups must stop in order to obtain the noise estimate,thereby adding to the overall cycle time of seismic surveyingoperations.

SUMMARY OF THE INVENTION

The present invention pertains to a method and apparatus for noiseestimation. The method including producing a first chirp signal having avarying frequency over a first time period, sampling noise energy at afrequency different than the first chirp signal frequency during thefirst time period, and generating a noise estimate based on the noiseenergy sampled.

Additional features and advantages of the present invention will beapparent from the accompanying drawings and from the detaileddescription that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1A illustrates a prior art sequential sweep operation that uses abroad band energy detector to estimate noise energy.

FIG. 1B illustrates a prior art slip sweep operation that uses a broadband energy detector to estimate noise energy.

FIG. 2A is a flow diagram illustrating one embodiment of a noiseestimation scheme.

FIG. 2B is a flow diagram illustrating another embodiment of a noiseestimation scheme.

FIG. 2C illustrates a frequency-time spectrum of vibroseis operationaccording to one embodiment of a noise estimation scheme.

FIG. 3 illustrates a frequency-time spectrum of vibroseis operationaccording to another embodiment of a noise estimation scheme.

FIG. 4 illustrates one embodiment of a noise estimation scheme in slipsweep operations.

FIG. 5A illustrates a system utilizing one embodiment of a noiseestimation scheme.

FIG. 5B illustrates one embodiment of a noise estimator.

FIG. 6 illustrates another embodiment of a noise estimator.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forthsuch as examples of specific times, frequencies, process steps,components, etc. in order to provide a thorough understanding of thepresent invention. It will be apparent, however, to one skilled in theart that these specific details need not be employed to practice thepresent invention. In other instances, well known circuits or methodshave not been described in detail in order to avoid unnecessarilyobscuring the present invention.

A noise estimation system and method wherein noise is sampledapproximately concurrent with chirp signal sweeps are described. In oneembodiment, the noise estimation circuit and method described herein maybe implemented with a seismic surveying operation. When used with aseismic surveying operation, the cycle time of seismic surveyingoperations may be reduced by providing for noise estimation withoutadding deadtime to the surveying operations. It should be noted,however, that the present invention is described in relation to aseismic surveying only for illustrative purposes and is not meant to belimited to noise estimates in seismic surveying as the present inventionmay be applied to other frequency variant signal environments.

In a typical surveying operation, a group of vibroseis trucks proceedsto a vibrator point (VP) and each truck lowers a pad used to generate achirp sweep. The trucks sweep chirp signals, wait for the end of thelistening period, and sweep again. This procedure is repeated until therequisite number of sweeps for the VP is completed. The vibroseis trucksin the group then lift their pads, move to the next point, and repeatthe sequence at the new VP. In order to obtain an in-field estimate ofthe data quality, a passive measurement of the ambient noise level ismade. If the ambient noise level is too high, then surveying operationsmay have to be halted until the ambient noise level falls to anacceptable SNR level.

FIG. 2A is a flow diagram illustrating one embodiment of a noiseestimation scheme. The noise estimation scheme described herein may beused to perform noise measurement in a time interval that isapproximately concurrent with a time interval that a chirp signal istransmitted. A chirp signal having a swept frequency during a first timeperiod is produced, step 201. Noise energy is sampled during the sametime period but at a different frequency than the chirp sweep at anygiven point in the time period, step 203.

In one embodiment, a desired signal is monitored during a listeningperiod. The chirp sweep and listening period occur in the same timeperiod as the noise measurement but at frequencies that are offset fromthe noise measurement frequency. As such, noise energy is sampled duringthe same time period but at a different frequency than the chirp sweepat any given point in the time period. In this manner, the noisemeasurements may be performed while the chirp sweep and listeningperiods are proceeding. A noise estimate may be generated based on thenoise measurements, step 204.

FIG. 2B is a flow diagram illustrating another embodiment of a noiseestimation scheme. In one embodiment, a chirp signal having anincreasing frequency in a time period is generated, step 206. Thefrequency is increased within a frequency range having a start frequencyand an end frequency. Reflected seismic signals are listened for in thesame frequency range as the chirp signal transmission, step 207. Thelistening time period for the reflected signals is approximatelyconcurrent with the chirp signal generation time period, but with an endtime that is offset from the end time of the chirp signal generation bya predetermined amount. A subsequent predetermined time after the starttime of the listening period, noise energy measurement is initiatedbeginning at the same start frequency as the chirp signal, step 208.

In this manner, the noise energy measurements are made in the samefrequency range and approximately concurrent with the chirp signalsweep; however, during any given point in time, noise is measured in afrequency range different than the frequency of the chirp signal and thefrequencies of the echoes. A noise estimate is then generated based onthe sampled noise energy, step 209. It should be noted that thefrequencies and times used in the embodiments described herein are forillustrative purposes only, and that other frequencies and times may beused.

FIG. 2C illustrates a frequency-time spectrum of vibroseis operationaccording to one embodiment of a noise estimation scheme. Chirp signal210 is an acoustic energy wave that is linearly increased in frequencyfrom f₀ to f₁ over a period of time from t₀ to t₃. In one embodiment,for example, chirp signal 210 has a frequency that is linearly increasedfrom 5 to 150 Hertz (Hz) during a 15 second time period. Echoesreturning to the surface will be delayed by the travel time through thesubterranean layers. The earliest received echo will be from theshallowest layer and will have a frequency content corresponding to thelowest chirp frequency f₀. The listening time may start immediatelyafter the start t₀ of chirp signal 210 with echoes being received out toa time t₁ corresponding to the travel time of the acoustic waves to andfrom the deepest layers of interest. The listening time window has apredetermined time interval Te that the seismic detectors will listenfor a given frequency resulting in echo region 220. In one embodiment, abroad band filter is used to screen the reflected waves in echo region220 between f₀ and f₁ until time t₄.

Passive noise estimation 230 is performed using a sliding time-variantnarrow band filter in a frequency-time domain 230 that is delayed withrespect to chirp signal 210 and echo region 220. In one embodiment, thenarrow band filter is a bandpass filter. In another embodiment, thenarrow band filter is a combination of a high pass filter and a low passfilter.

The noise measurement begins at a time just after the end of thelistening time period t₁ for frequency f₀ and lasts for a predeterminedtime Tn, until time t₂. The frequency at which noise energy is measuredis delayed with respect to echo region 220 and linearly increases at arate equal to or less than the rate of the chirp sweep so that noisemeasurement in one frequency band may be performed while a chirp issweeping in a different frequency band. In this manner, noise estimationmay be performed concurrently with sweep operations because thefrequency-time domain of noise measurement region 230 is outside echoregion 220. Furthermore, a new chirp signal 250 may be started as soonas the listening period of the first chirp signal 210 ends, at time t₄.The new chirp signal 250 may begin at time t₄ even though noisemeasurement is still in progress, because the time-variant narrow bandfilter has been swept to evaluate noise in a frequency range f₂ to f₁while new chirp signal 250 is being swept at the frequency range f₀ tof₄ in the same time period t₄ to t₅.

Using the sliding time-variant narrow band filter described hereineliminates the deadtime required when using a broad band noise estimatorfunctioning outside of a sweep and listening time period. By performingnoise estimation during a sweep and listening period, the time betweenconsecutive sweeps (cycle time 260) may be reduced because a new sweepmay begin immediately after the end of a prior sweep. Therefore, thetotal cycle time of surveying operations may be reduced.

For example, if chirp signal 210 has a sweep time of 15 seconds (t₃=15seconds), a listening sweep time of 5 seconds (t₄−t₃=5 seconds), and anoise estimation sweep time of 2 seconds (t₅−t₄=2 seconds), then cycletime 260 is only 20 seconds (t₄=20 seconds). This is approximately a 9%reduction in cycle time from the 22 second cycle time of operationsusing the prior art noise estimation technique shown in FIG. 1A.

It should be noted that although FIG. 2C illustrates chirp signal 210and echo region 220 with a linearly increasing frequency over time, thechirp signal and listening region may also be linearly decreased withthe noise region 230 correspondingly varied. In an alternativeembodiment, the frequencies of the chirp signal 310, echo region 320,and noise region 330 may be varied over time in other manners, forexample, exponentially as illustrated in FIG. 3. It should also be notedthat noise estimation scheme described herein may also be used in slipsweep operations as shown in FIG. 4.

FIG. 4 illustrates one embodiment of noise estimation in slip sweepoperations. In slip sweep operations, multiple groups of vibroseistrucks sweep in non-overlapping frequency-time windows such thatdifferent groups are not sweeping in the same frequency at the sametime. Noise measurement in noise region 430 is performed using a slidingtime variant narrow band filter in a frequency-time domain that isdelayed with respect to chirp signal 410 and echo region 420. In oneembodiment, the narrow band filter may be a bandpass filter. In anotherembodiment, the narrow band filter may be a combination of a high passfilter and a low pass filter.

The noise measurement for frequency f₄₀ of noise region 430 begins at atime just after the end of the echo region (listening time period) t₄₁and lasts for a predetermined time Tn₄, until time t₄₂. The frequencyfor which noise measurement is performed may be delayed in time withrespect to the frequencies received in echo region 420 and may belinearly increased at a rate approximately equal to the rate of chirp410. This allows noise measurement to be performed concurrently with thelistening period defined by echo region 420. The overlapping duration ofthe echo region and the noise measurement is possible because the noisefor any given frequency is measured at time delayed from the receipt ofreflected seismic waves for the same frequency. In another embodiment,the frequency at which noise measurement is performed may be linearlyincreased at a rate less than the rate of chirp 410.

A chirp signal 450 for a subsequent vibroseis truck group is started atfrequency f₄₀ at t₄₂ as soon as the time period Tn₄ for noisemeasurement at frequency f₄₀ has ended. A time variant sliding narrowband filter may be used to separate chirp signals 410 and 450. In thismanner, noise estimation may be performed because the frequency-timedomain of noise measurement region 430 is outside of chirp signal 410,echo region 420, chirp signal 450, and the echo region 460 for chirpsignal 450. It should be noted that the frequency of the chirp signalsweeps and the corresponding noise measurements in slip sweep operationsmay also be non-linearly varied with time, for example, exponentially.

FIG. 5A illustrates a system utilizing one embodiment of a noiseestimation scheme. The system includes a chirp signal generator 501, anda signal detector and a noise estimation circuit 502. The chirp signalgenerator 501 has a control input 504 that is used to generate afrequency swept signal over time. In one embodiment, timing control 505can be used to synchronize the operation of chirp signal generator 501with signal detector and noise estimation circuit 502. The detector 502includes input circuitry to detect acoustic energy and generate internalsignals having frequency components to be used by a frequency filter.

FIG. 5B illustrates one embodiment of a noise estimator. The noiseestimator 500 is used to measure noise concurrently with sweepingoperations and generate a noise estimate. The noise estimate may be usedto determine if sweeping operations should be halted until the ambientnoise falls to allow for clear reception of echoes. In one embodiment,the noise measurements include reflections from subterranean layersdeeper than those of interest.

In one embodiment, noise estimator 500 includes a geophone 510, anamplifier 520, a mixer 540, a voltage controlled oscillator (VCO) 550, abandpass filter 570, a rectifier 580, and an integrator (∫dt) 590. Itshould be noted that a geophone, an amplifier, a mixer, a VCO, abandpass filter, a rectifier, and an integrator are well known in theart; accordingly, a detailed description of their internal componentsand their operation is not provided herein.

The noise estimator 500 calculates an estimate of the noise energy basedon the noise measurements made throughout the frequency range ofoperation. Acoustic energy waves of noise are received at geophone 510and converted into an analog electrical signal 511 that characterizesthe magnitude and frequency content of the acoustic energy waves. Theelectrical signal 511 is amplified by amplifier 520 and transmitted tomixer 540. The output 531 of amplifier 520 is combined with VCO 550output signal 551 by mixer 540. VCO 550 is used to slide signal 531 sothat noise estimation can be performed at different frequencies whilekeeping the band range of bandpass filter 570 fixed. Bandpass filter 570is used to screen out frequencies, in a particular time domain, forwhich noise measurement is not desired. In one embodiment, bandpassfilter 570 may remove unwanted negative signal images from previouslower frequencies that have been shifted up into a higher frequencyrange.

In one embodiment, for example, the chirp signal is swept in frequencyfrom 10 Hz to 150 Hz while the frequency of VCO 550 is decreased from390 Hz to 250 Hz. By mixing the amplified sensor signal 531 with outputsignal 551 of VCO 550, the output 541 of mixer 540 will haveapproximately a constant frequency of 400 Hz. The mixer differencefrequencies between the chirp signal and the VCO range from 380 Hz to100 Hz. As such, bandpass filter 570 may be selected to have a bandpassfrequency centered at 400 Hz with approximately a +/−20 Hz bandwidth inorder to filter out these mixer difference frequencies.

In an alternative embodiment, the signal representing the acousticenergy waves of noise is fixed and the band range of bandpass filter 570is adjusted based on a sliding time. The VCO 550, mixer 540, and filter570 are replaced with a tunable digital bandpass filter having taps thatare changeable based on a run time.

FIG. 6 illustrates another embodiment of a noise estimator. In thisembodiment, noise estimator 600 includes a geophone 610, an amplifier620, a digitizer 630, a fast fourier transform circuit (FFT) 645, acomplex conjugate multiplier circuit 665, a multiplexer 675, and adigital summation circuit 690. It should be noted that a geophone, anamplifier, a digitizer, a fast fourier transform circuit, a complexconjugate multiplier circuit, a multiplexer, and a digital summationcircuit are well known in the art; accordingly, a detailed descriptionof their internal components and their operation is not provided herein.

The digital signal characterizing the magnitude and frequency content ofthe noise energy is applied to FFT 645. FFT 645 functions as a bank ofnarrow band filters and generates an output 646 in the form of a complexnumber. The magnitude of output 646 is squared 665 to generate anestimate of the power spectral density. A multiplexer 675 is used toselect from among the different frequency bins 667 of the power spectraldensity. The output of the multiplexer 675 is applied to digitalsummation circuit 690 to generate the noise estimate 691.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

What is claimed is:
 1. A method of noise estimation, comprising thesteps of: producing a first chirp signal at a varying first chirpfrequency within a frequency band during a first time period, whereinthe first chirp signal results in first chirp echoes, which are observedat first chirp echo frequencies during the first time period; takingnoise measurements during the first time period at a varying first noiselistening frequency within the frequency band, wherein at any point intime during the first time period, the first noise listening frequencyis different from both the first chirp signal frequency and the firstchirp echo frequencies at said point in time during the first timeperiod; and generating a noise estimate based on the noise measurements.2. The method of claim 1, further comprising the steps of: producing asecond chirp signal at a varying second chirp frequency within thefrequency band during a second time period, wherein the second timeperiod begins at a later point in time than the first time period,begins, and the second chirp signal results in second chirp echoes,which are observed at second chirp echo frequencies during the secondtime period; and taking noise measurements during the second time periodat a second varying noise listening frequency within the frequency band,wherein at any point in time during the second time period, the secondnoise listening frequency is different from the first chirp signalfrequency, the second chirp signal frequency, the first chirp echofrequencies, and the second chirp echo frequencies at said point in timeduring the second time period.
 3. The method of claim 2, wherein thefirst and second varying chirp frequencies vary linearly with time. 4.The method of claim 2, wherein the first chirp frequency and the secondchirp frequency vary exponentially with time.
 5. The method of claim 3wherein the first noise listening frequency and the second noiselistening frequency vary linearly with time.
 6. The method of claim 3wherein the first noise listening frequency and the second noiselistening frequency vary exponentially with time.
 7. A method of ambientnoise estimation, comprising the steps of: producing a first chirpsignal at a varying first chirp frequency within a frequency band duringa first time period; taking a first set of ambient noise measurements ata varying first noise listening frequency within the frequency bandduring the first time period, wherein at any point in time during thefirst time period the first noise listening frequency is different fromthe first chirp frequency at said point in time during the first timeperiod; and generating an estimate of ambient noise within the frequencyband based on the first set of ambient noise measurements.
 8. The methodof claim 7 further comprising the steps of: producing a second chirpsignal at a varying second chirp frequency within the frequency bandduring a second time period, wherein the second time period begins at alater point in time than the first time period; taking a second set ofambient noise measurements at a varying second listening frequencywithin the frequency band during the second time period, wherein at anypoint in time during the second time period the second noise listeningfrequency is different from the first chirp frequency, the second chirpfrequency, and the first noise listening frequency at said point in timeduring the second time period; and generating an estimate of ambientnoise within the frequency band based on the second set of ambient noisemeasurements.
 9. The method of claim 8 wherein the first chirp frequencyand the second chirp frequency vary linearly with time.
 10. The methodof claim 8 wherein the first chirp frequency and the second chirpfrequency vary exponentially with time.
 11. The method of claim 8wherein the first listening frequency and second listening frequencyvary linearly with time.
 12. The method of claim 8 wherein the firstlistening frequency and second listening frequency vary exponentiallywith time.